Quartz crystal sensor cell

ABSTRACT

A quartz crystal sensor cell in which the sensor is secured to the base of a well defining a test sample space by a layer of adhesive between the respective peripheral surfaces. Also a quartz crystal sensor cell is provided in which the quartz sensor crystal is inclined relative to the opposite surface of the test sample space. These constructions respectively improve distinctness of the “Q” peak and problems from reflected waves in the space.

This invention relates to improvements in cells for quartz crystalsensors.

The quartz crystal sensor is a mass sensing device with the ability tomeasure very small mass changes on the surface of a piezoelectric quartzcrystal. A device using such a quartz crystal sensor is generally knownin the art as a quartz crystal microbalance. The quartz crystal ismounted in a cell in which materials under test can be exposed to thesurface of the crystal or on a receptor coated on the crystal surface.Quartz crystal sensors were originally developed to detect deposits onthe crystal sensor under vacuum conditions. When the crystal is in agaseous environment and is energised by an oscillator circuit there is awell-defined relationship between frequency shift and a mass depositedon the crystal surface. In liquid environments the density and viscosityof the liquid and visco-elastic properties of the receptor affect theresponse. Mass changes as small as the deposit of a single layer ofatoms can be detected.

Further developments to measure deposits from liquid carriers flowingthrough the cell have needed to face problems caused by damping of thesensitivity of the sensor due to the liquid in contact with the crystal.This is addressed in our patent application PCT/EP99/08148 whichproposes novel circuitry to drive the crystal.

In our continuing study of quartz crystal sensors, we have found thatfurther problems in sensitivity and reproducibility of results arisefrom the physical configuration of existing cells. A typicalconfiguration of a cell is shown in FIG. 1 of the accompanying drawings.In this cell, a sample space (1) is formed by clamping a quartz crystal(2) between upper and lower housing parts (4) and (3); for example bypassing bolts through bore holes (5). The upper housing part (4) isformed with a cylindrical depression or “well” so that the sample space(1) is delimited by the upper surface of the quartz crystal (2), thesidewalls (6) of the depression and an upper surface (7) bridging thesidewalls. Inlet and outlet ports (8,9) formed in the upper housing part(4) allow a fluid carrying a sample to pass into and out of the samplespace. Electrodes (10) and (11) on opposite surfaces of the quartzcrystal (2) allow the crystal to be driven by an oscillator circuit.

Conventionally the quartz crystal (2) is secured between the upper andlower body parts (4) and (3) by resilient O-rings (12) and (13) whichabsorb some of the clamping pressure and seal the sample space. Inpractice, this arrangement results in poor reproducibility of repeatedsampling because of variations from run to run in the torque applied inclamping the sensor in the cell. This can cause variations in crystalalignment and even cracking of the crystals. The variations incompression of the crystal and in alignment result in an indistinct peak“Q” value, the parameter by which the frequency change is measured.

We have found that this problem can be overcome by constructing a cellin which the crystal sensor is secured directly to the housing.

Accordingly in a first aspect the present invention provides a quartzcrystal sensor cell comprising a housing having a well to receive asample carrier fluid, an aperture in the base of the well to receive aquartz crystal sensor, and a quartz crystal sensor located in the baseof the well so that a peripheral surface of the sensor lies against aperipheral surface of the aperture, and the sensor is secured to thebase of the well by a layer of adhesive between the respectiveperipheral surfaces.

The base of the cell may consist of a peripheral flange directedinwardly from the side walls of the well to define a central aperture.Suitably, the well is formed as a cylindrical bore having an internalannular flange to form the apertured base of the well. The sensor istypically circular in shape and has a diameter greater than the diameterof the central aperture defined by the annular flange.

The sensor may be adhered to the flange by forming a continuous bead orlayer of curable adhesive on the flange, and placing the sensor on thebead while the adhesive cures, to form an adhesive layer between therespective overlapping edges of the sensor and flange. Preferably, thecurable bead of adhesive is a flowable composition and the sensor isallowed to settle on the bead under its own weight while the bead curesas a thin interlayer. Suitably the sensor is located on the underside ofthe flange relative to the void space of the well, requiring that thecell is inverted as the sensor is applied. Non-solvent-based adhesivesare preferred to avoid problems with solvent removal. However, asolvent-based adhesive could be used to deposit a pressure sensitiveadhesive layer.

The sensor may be secured non-compressively by any adhesive. However,use of a rigid-setting adhesive may itself impose stresses on thesensor. Therefore, the adhesive is preferably selected so as to cure toform a resilient interlayer between the sensor and the flange. Siliconepolymers have been found to be especially suitable.

The cell may be closed by a housing part positioned across the mouth ofthe well. Preferably this closure has inlet and outlet ports enabling afluid sample carrier to pass through the cell. In this configuration thecell can be used as a flow cell, to measure dynamic characteristics as asample is deposited on the sensor. In particular, such cells may be usedto investigate the binding characteristics between a sample in thecarrier fluid and a substrate pre-deposited on the sensor.

It is not always necessary that a quartz crystal sensor cell is operatedas a flow cell. In some situations, the analytical requirements aresimply to detect a binding event between a substrate deposited on thesensor surface and a test material suspended or dissolved in the carrierliquid. In that case, it is not necessary for the closure part to beprovided with inlet and outlet ports and connectors for attaching feedtubing. Instead, in an alternative embodiment, the closure may beprovided with a single port, which may be simply a centrally locatedaperture or bore hole, through which a sample carrier liquid can beinjected into the well to contact, preferably covering the sensor.

An array of such cells may be used with an automated sample injectorsystem. In this configuration the sensor can be used simply to detectbinding events when dynamic information is not necessary.

In a particular embodiment, multiple cells are formed as bore holes in arectangular housing block, for example, in a standard 96 cell array thatmay be used with a conventional automated sample injector. Each well hasa sensor adhered to a peripheral flange forming its base. In thisembodiment, the wells may be fully open without any closure.

Our study of quartz crystal microbalance flow cells has also revealedthat reflected waves occur between the crystal and the substantiallyparallel upper boundary surface of the sample space. We have discoveredthat this problem may be overcome by use of an asymmetric sample space.

Accordingly, in a second aspect the present invention provides a flowcell for a quartz crystal sensor comprising a housing, a quartz crystalmounted in the housing to form one boundary surface of a sample spacedelimited by the housing internal wall surfaces and an upper housingsurface bridging the housing walls, and having inlet and outlet ports onthe housing for conveying a sample carrier into and out of the samplespace, characterised in that the upper housing surface is inclined tothe plane of the quartz crystal.

This construction of flow cell of this second aspect of the inventionmay be used with flow cells of otherwise conventional construction, e.g.with their quartz crystal secured between a pair of upper and lowerresilient O-rings. Preferably it is used with the flow cell of the abovedescribed first aspect of the invention with its sensor secured to thebase of a well by a layer of adhesive.

By “inclined” we mean generally non-parallel. We have found that if theinclination of the upper boundary surface is too steep, bubbles tend toform in the sample space. Accordingly, the angle of inclination must beselected so as to balance the effect of minimising both the occurrenceof reflected waves and the formation of bubbles within the sample space.We have found that an angle of inclination between 1–5° is suitable,preferably between 1–3°, about 2° (e.g. + or −0.5°) being effective.

Preferably the angle of inclination is such that the upper boundarysurface rises relative to the crystal sensor, between the inlet andoutlet ports. When the inlet and outlet ports are formed in the upperboundary surface, then preferably the outlet is at or near the highestpoint of the sample space i.e. where there is maximum separation betweenthe upper housing surface and the crystal.

When intended for use as a flow cell, the cell housing is convenientlyformed in two parts, a first (lower) housing part having a well with acentral aperture in which the crystal sensor is secured to form a lowerboundary surface of the sample space, and a second (upper) housing partwhich is located on the lower part over the well and crystal sensor toform an upper boundary surface of the sample space.

The well is suitably a hollow cylindrical void having an internalannular flange to form the base. The sensor is typically circular inshape and has a diameter greater than the diameter of the centralaperture defined by the annular flange.

Typically when the upper housing part is clamped to the lower housingpart it engages a resilient O-ring to seal the sample space. Suitablythe O-ring is seated against a peripheral flange in the well and thesensor is adhered to the opposite side of the flange. This arrangementminimises the transfer of clamping forces to the crystal.

The inlet and outlet ports for the flow cell are most convenientlyprovided in the upper housing part, most suitably opening into thesample space at the upper boundary surface. It is preferred that theports are oriented so that the sample carrier enters and leaves thesample space in a direction substantially normal to the surface of thequartz crystal, though for some practical situations alternativeorientations may be preferred.

For easy assembly and maintenance it is advantageous that the sensor ismounted in a separate carrier block comprising the well and flange forseating the sensor. The carrier block is located in a base block towhich the upper housing part is bolted as a closure for the well.

The present invention is described in more detail below with a referenceto the accompanying drawings which show, by way of example only:

FIG. 1 is a schematic cross-section through a prior art flow cell;

FIG. 2 is a schematic cross-section through a flow cell in accordancewith both aspects of the present invention;

FIG. 3 is a cross-section through a practical embodiment of a flow cellin accordance with both aspects of the present invention; and

FIGS. 4( a) and (b) are respectively partial sectional and plan views ofa multiple static cell unit.

FIG. 1 shows a prior art construction already described above. Asmentioned previously, we have found that elements of this design areunsatisfactory from the point of view of reproducibility of results anddistinctness of the “Q” value peak. These arise inter alia from theclamping of the crystal sensor between housing parts (3) and (4) viaresilient O-rings (12) and (13).

FIG. 2 shows (schematically and not to scale) a flow cell constructed inaccordance with the findings of the present invention.

The cell has a two-part structure comprising a lower (base) body block(20) and an upper (closure) body block (21) which together form ahousing. The lower body block (20) is formed to define a well (22),which is a cylindrical void having an aperture (23) in its base, andsurrounded by a peripheral flange (24) which serves as a seat for aquartz crystal sensor (25). The quartz sensor (25), a thin wafer ofquartz, is secured to the lower surface of the flange (24) by anadhesive layer (26). Suitably this is achieved by use of a curablesilicone adhesive which cures in air to form a resilient layer. Thisallows the crystal (25) to be directly secured to the flange (24)without the risk of applying uneven or non-reproducible stresses to thecrystal sensor. By inverting the lower block (20) and injecting acontinuous bead of adhesive (26) against the flange (24), then allowingthe sensor (25) to rest against the bead under the influence of gravityonly, the layer of adhesive (26) attaches the sensor (25) to the flange(24) without imposing external stresses.

To enclose a sample space in the well (22), the upper body part (21) islocated against the lower body part (20), effectively forming a closureto the well (22), so that a sample space is delimited by the side walls(27) of the well (22), the upper surface of the crystal (25) and theunder surface (28) of the upper block (21).

In accordance with the present invention, this surface (28) is inclinedso that it is not parallel with the crystal sensor (25) when the upperand lower body blocks (20), (21) are mated together. We have found thatan angle of inclination of around 2° gives good results, providing abalance between the reduction of the effects of reflected waves, and theformation of bubbles which occur as the angle of inclination isincreased.

To ensure sealing of the sample space in the well (22), the upper bodyblock (21) is shaped to provide an annular projection (29) whichprotrudes into the well (22) to engage a resilient O-ring seal (210),which conveniently rests on the upper surface of the flange (24). Theannular projection (29) surrounds the surface (28) of upper block (21)forming a recess which defines the upper part of the sample space in thewell (22).

The two body parts (20, 21) may be secured to each other, compressingthe O-ring seal (210), by bolts (not shown) passed through bolt holes(211) formed in the blocks (20,21).

The upper body block (21) is also formed with bore holes (212) and (213)acting respectively as inlets and outlets for a liquid carrier whichconveys test material into the sample space in the well (22). Preferablythese bore holes (212, 213) are arranged so that the carrier enters andexists from the sample space in a direction substantially at rightangles to the crystal sensor (25), although their orientation may needto be varied to suit the external connections to tubing by which thecarrier liquid is fed and removed.

Electrodes (214) are positioned on each surface of the crystal sensor(25) so that the piezoelectric crystal can be driven by an externaloscillator circuit. The electrodes may be connected to a conventionaloscillator circuit, or more preferably an oscillator circuit asdescribed in PCT/EP99/08148, by silver conductive paint leads (notshown) carried from the electrodes around the edges of the crystal toconductors inserted into the housing.

Having described the principles of this invention with respect to theschematic embodiment of FIG. 2, we now refer to the practical embodimentshown in FIG. 3 of the accompanying drawings in which theabove-described principles are implemented.

FIG. 3 shows a sectional view through a quartz crystal sensor flow cell.In this embodiment, quartz crystal sensor (40) is secured to a carrierblock (41) which locates in a base block (48). The combined carrier (41)and base block (48) are equivalent to the lower housing part (20) ofFIG. 2. In an analogous manner to the FIG. 2 embodiment, the carrierblock (41) comprises a cylindrical well (42) and a circular aperture(43) in its base, and being surrounded by a peripheral flange (44). Theflange (44) has two portions: a relatively thick root portion (44 a),adjacent the walls of well (42) which supports an O-ring (45); and arelatively thin extension (44 b) which provides a flange surface toreceive the sensor (40). As in FIG. 2, the sensor (40) is adhered to theunderside (relative to the sample space of the well) of the flange (44b), sitting in a reversed well (46) defined by the edges of the flangeroot (44 a).

Conductive sockets (47) are positioned in bores in the housing of thecarrier (41) and the sockets (47) are connected by conducting leads tothe electrodes of the sensor (40), when the latter is adhered to theflange (44 b). The sensor carrier block (41) is located, via theconductive sockets (47), on conductor pins (48) protruding into a well(49) formed in the base block (410). The conductor pins (48) are in turnconnected to the oscillator circuit (not shown) which drives the crystalsensor (40).

A closure part (411) bolts on to the base (410) through bolt holes (412b, 412 b) respectively in the closure (411) and in the base (410). Theclosure (411) has a protruding portion (413) which extends into the well(42) of the carrier block (41) when the closure (411) is secured to thebase (410). The forward edge (413 a) of protrusion (413) is brought intocontact with the flange root (44 a), while a bevelled side edge (413 b)engages O-ring (45). A recess (414) within protrusion (413) defines asample space in the well (42) above the sensor (40). Inlet and outletbores (415, 416) formed in the body of the closure (411) allow a carrierfluid to be passed into the sample space to bring test materials intocontact with the sensor (40).

As in FIG. 2, the upper surface of the recess (414) is inclined,typically at ca. 2°, to the plane of the sensor (40), and the outlet(416) is positioned at the highest point of the recess (414).

In the embodiment shown, the inlet and outlet bores (415, 416) areinclined to the central axis of the closure (411) to increase theseparation of the ports on the upper surface of the closure (411). Thisgives space for positioning of known HPLC connector bushes in the bores(415, 416). This allows HPLC tubing to be used to flow the carrierliquid in to and out of the cell.

In an alternative embodiment (not shown), for example for situationswhere the analytical requirements are simply to detect a binding eventbetween a substrate deposited on the sensor surface and a test materialsuspended or dissolved in the carrier liquid, it is not necessary forthe closure part (411) to be provided with inlet and outlet ports andconnectors for attaching feed tubing. In this alternative embodiment,the closure (411) is provided with a single port, which may be simply acentrally located aperture or bore hole, through which a carrier liquidcan be injected into the well (42) to cover the sensor (40).

Further, when the quartz crystal sensor cell is not a flow cell, it canbe greatly simplified. For example, the cell can be an open well orpreferably an array of open wells as shown in FIGS. 4 a and 4 b. In theembodiment of FIG. 4, a series of wells (60) are formed in a base block(70) in a regular array, suitably 12×8. Each well (60) has an inwardlydirected flange towards one end. The well is closed by adhering a quartzcrystal sensor (62) to each flange (61) using an adhesive which providesa resilient interlayer (63). The base of the block (70) is provided withelectrical connections to power each crystal individually and detect the“Q” output. The base block (70) is suitably dimensioned to be used withexisting automated sample injector apparatus whereby a carrier liquidand samples are injected into each well (60), so that binding events maybe detected and analysed rapidly and economically on multiple samples.

It is desirable to control the temperature of the samples being tested,in both flow and static testing. An advantage of locating the sensor onthe underside of the peripheral flange at the base of each well is thatthe sensor can be positioned against a Peltier element for temperaturecontrol.

A typical quartz crystal sensor (24, 40) is a circular component ofdiameter of about 8.6 mm. For optimum sensitivity the sample space abovethe crystal sensor is designed to be as small as possible and in a flowcell a suitable volume is around 20 micro liters. The inlets and outletsare designed to allow the desired flow volume, typically 1 to 100 microliters per minute. The body parts may be machined or moulded fromplastics material preferably a biocompatible polymer such aspolyethylethylketone (PEEK) which is non-adherent to proteins, becausemany applications for quartz crystal microbalances of this type involveassaying binding properties between biological compounds.

When the sensor is secured to a separate carrier (41), then the carrieris also suitably formed from PEEK, allowing the base block (410) to beformed from metal, such as anodised aluminium. In that case theconductive pins (48) are mounted in non-conductive bushes (55).

In a typical use for the cells described above, the quartz crystalsensor (24, 40) is coated with one element of a coupling combination(e.g. a first chemical or biological compound), a sample known orbelieved to contain the other component of the coupling combination(e.g. a second chemical or biological compound) is passed through thecell using an aqueous carrier, the quartz crystal is driven by anoscillator circuit, and changes in mass of the deposit on the sensor aredetermined by the frequency change and/or the “Q” value of the drivencrystal. These changes may be used to interpret the coupling activity ofthe compounds under test.

1. A quartz crystal sensor cell comprising a housing, a well in said housing for receiving a sample carrier fluid, said well having a surrounding side and a base with an aperture, said aperture having a peripheral surface, a quartz crystal sensor having a peripheral surface overlapping the peripheral surface of the aperture, and a layer of adhesive between the respective peripheral surfaces, securing said peripheral surfaces to each other and thereby securing the quartz crystal sensor to the base, in which the base of the well comprises a peripheral flange extending inwardly from said surrounding side of the well, said flange defining said aperture, and in which the adhesive layer forms a resilient interlayer between the sensor and the flange.
 2. A cell as claimed in claim 1, in which the well is in the form of a cylindrical bore in said housing, in which said flange is annular in shape and said aperture is circular, and in which the sensor is circular in shape and has a diameter greater than the diameter of said aperture defined by the flange.
 3. A cell as claimed in claim 1, in which said well is in the form of a cylinder having said base at one end thereof and being open at an end opposite said one end.
 4. A cell comprising a housing, a well in said housing for receiving a sample carrier fluid, said well having a surrounding side and a base with an aperture, said aperture having a peripheral surface, a quartz crystal sensor having a peripheral surface overlapping the peripheral surface of the aperture, and a layer of adhesive between the respective peripheral surfaces, securing said peripheral surfaces to each other and thereby securing the quartz crystal sensor to the base, and including a closure extending across said well at a location spaced from said sensor.
 5. A cell as claimed in claim 4, in which said closure has inlet and outlet ports enabling a fluid sample carrier to pass through the cell.
 6. A cell as claimed in claim 4, in which said closure has an aperture allowing a sample carrier fluid to be injected into the well to contact the sensor.
 7. A quartz crystal sensor cell comprising a housing, a well in said housing for receiving a sample carrier fluid, said well having a surrounding side and a base with an aperture said aperture having a peripheral surface, a quartz crystal sensor having a peripheral surface overlapping the peripheral surface of the aperture, and a layer of adhesive between the respective peripheral surfaces, securing said peripheral surfaces to each other and thereby securing the quartz crystal sensor to the base, in which said housing comprises lower and upper housing parts, said lower housing part having said well and said base, in which said aperture is a central aperture in said base, in which the crystal sensor forms at least a part of a first boundary surface of a sample space within said housing, and said upper housing part is located on the lower part, over said well, and in spaced relation to said crystal sensor, said upper housing part forming an upper boundary surface of said sample space.
 8. A cell as claimed in claim 7, in which the aperture in said base is defined by a flange having upper and lower sides, said crystal sensor is adhered to the lower side of said flange, a resilient O-ring is seated against the upper side of said flange, said upper housing part is secured to said lower part, and said O-ring is clamped between said upper housing part and the upper side of the flange.
 9. A cell as claimed in claim 7, in which the lower housing part comprises a base block and a carrier block, said aperture is formed in said carrier block, said crystal sensor is mounted on said carrier block, said carrier block is received in said base block, and said upper housing part is bolted to the base block and forms a closure for said well.
 10. A quartz crystal sensor cell comprising a housing, a well in said housing for receiving a sample carrier fluid, said well having a surrounding side and a base with an aperture, said aperture having a peripheral surface, a quartz crystal sensor having a peripheral surface overlapping the peripheral surface of the aperture, and a layer of adhesive between the respective peripheral surfaces, securing said peripheral surfaces to each other and thereby securing the quartz crystal sensor to the base, in which said housing has an internal sample space defined in part by a surrounding side wall surface, and an upper housing surface forming an upper end boundary of said sample space, said quartz crystal sensor has a planar face forming at least a part of a lower end boundary of said sample space, and in which said upper housing surface is inclined relative to said planar face of the quartz crystal sensor.
 11. A quartz crystal sensor cell according to claim 10, in which said upper housing surface is inclined relative to said planar face of the quartz crystal sensor at an angle in the range from about 10° to 50°.
 12. A quartz crystal sensor cell according to claim 10, in which said upper housing surface is inclined relative to said planar face of the quartz crystal at an angle of 2+/−0.5°.
 13. A method of use of a quartz crystal sensor cell comprising a housing, a well in said housing for receiving a sample carrier fluid, said well having a surrounding side and a base with an aperture, said aperture having a peripheral surface, a quartz crystal sensor having a peripheral surface overlapping the peripheral surface of the aperture, and a layer of adhesive between the respective peripheral surfaces, securing said peripheral surfaces to each other and thereby securing the quartz crystal sensor to the base, wherein the quartz crystal sensor is coated with one element of a coupling combination, a sample known or believed to contain the other component of the coupling combination is passed through the cell in an aqueous carrier, the quartz crystal sensor is driven by an oscillator circuit, and changes in mass of the deposit on the sensor are determined by at least one of the frequency change and the “Q” value of the driven crystal.
 14. A quartz crystal sensor cell comprising a housing having an internal sample space defined in part by a surrounding side wall surface, a quartz crystal mounted in the housing said quartz crystal having a planar face forming at least a part of a lower end boundary of said sample space, an upper housing surface forming an upper end boundary of said sample space, said housing having inlet and outlet ports for conveying a sample carrier into and out of said sample space, wherein said upper housing surface is inclined relative to said planar face of the quartz crystal sensor.
 15. A quartz crystal sensor cell according to claim 14, in which said upper housing surface is inclined relative to said planar face of the quartz crystal sensor at an angle in the range from about 1° to 50°.
 16. A quartz crystal sensor cell according to claim 14, in which said upper housing surface is inclined relative to said planar face of the quartz crystal sensor at an angle of 2+/−0.5°.
 17. A method of use of a quartz crystal sensor cell according to claim 14, wherein the quartz crystal sensor is coated with one element of a coupling combination, a sample known or believed to contain the other component of the coupling combination is passed through the cell in an aqueous carrier, the quartz crystal sensor is driven by an oscillator circuit, and changes in mass of the deposit on the sensor are determined by at least one of the frequency change and the “Q” value of the driven crystal.
 18. A quartz crystal sensor cell comprising a housing (20, 21, 41, 410, 411) having a well (22, 42) to receive a sample carrier fluid, an aperture (23, 43) in the base of the well (22, 43) to receive a quartz crystal sensor (25, 40), and a sensor (25, 40) located in the base of the well (22, 43) so that a peripheral surface of the sensor (25, 40) lies against a peripheral surface of the aperture (23, 43), and the sensor (25, 40) is secured to the base of the well (22, 42) by a layer of adhesive (26) between the respective peripheral surfaces, in which said layer of adhesive (26) forms a resilient interlayer between said respective peripheral surfaces.
 19. A quartz crystal sensor cell comprising a housing, a well in said housing for receiving a sample carrier fluid, said well having a surrounding side and a base with an aperture, said aperture having a peripheral surface, a quartz crystal sensor having a peripheral surface overlapping the peripheral surface of the aperture, and a layer of adhesive between the respective peripheral surfaces, securing said peripheral surfaces to each other and thereby securing the quartz crystal sensor to the base, in which said layer of adhesive forms a resilient interlayer between said respective peripheral surfaces. 